The rotary torque of an engine of a commercial car has in recent years been lowered at a low speed of the car because of the approach to reducing fuel consumption by reducing an exhaust gas and using a turbo system. Engine torque is increased when the car is started by adding an assist motor to the engine. FIG. 8 depicts the assist motor wherein a reference numeral 1 annotates a tire of a car, 2 annotates an engine, and 3 annotates the assist motor.
As shown in FIG. 8, the assist motor 3 is positioned in a limited space near the engine 2, so that it is necessary to reduce the assist motor in size. It is effective to increase the rotational speed of the motor more than 25000 rpm in order to make the motor small in size and high in power. However, in the conventional inverter, an output frequency is about 250 Hz and the maximum rotational speed is about 2500 rpm at the switching time. An object of the present invention is to obviate the above defects.